I was pondering the well known carrying items, like a magic chest, or bag of holding, and thought that the Ars Magica spell guidelines could be extended to have a new target called Container. It would function much the same as T: Room (a +2 mag Target), where all inhabitants of the room are affected.
Container – the spell affects everything within a container. This container must have an opening, have a easily discernible structure and boundaries, and be crafted specifically for carrying or moving. The base container may be of any size up to a coffin, large barrel, or cauldron (size +0 as base).
A key difference is the container does not have to be closed or locked to be valid, or even be water-tight as cups, bird cages, vases, sacks, or backpacks are examples of valid containers. A sack or pouch could not really be considered a room. Container may also be used as a valid target for Creo magic, allowing the mass crated to be up to the volume of the container, as long as the amount of material crated is large enough; thus an advantage over T: Room which cannot be a valid target for creo spells which create things.
With an increase in magnitude larger containers are allowed, such as +1 mag to allow affecting the contents of a massive chest, and +2 mag to affect the contents of a horse drawn cart.
The fundamental difference between using T: Room vs T: Container is the usage of the containers as typically being portable. Some hermetic spells may make use of T: Room to achieve T: Container in the setting, and I’d suggest this guideline be along side (although probably replacing it) for those effects.
As a minor hermetic breakthrough T: Container seems good enough to be worthy of a Bonisagus’s time, but still not so overt in power to be abusive. It would sit within the spell mechanics at the same degree of power as T: Room, and is intended as an extension to the regular targets already in the game, rather than as a target that would require a special virtue to use.